Functional analysis, properties and kinetics of a PepT2-type di/tripeptide transporter of the Atlantic salmon (Salmo salar) highly expressed in midgut and hindgut

The SoLute Carrier 15 (Slc15) family includes H+-dependent transporters that play a key role in the cellular uptake/reuptake of di/tripeptides and peptidomimetics. In mammals, in the epithelial cells of intestine and renal tubules two di/tripeptide transport systems have been characterized: the low-affinity/high-capacity system Slc15a1 (PepT1) and the high-affinity/low-capacity system Slc15a2 (PepT2). While PepT1 transporters have been studied in many teleost species, data on PepT2 is still lacking, except for zebrafish (Danio rerio). Here, we performed basic functional and expression analyses of a newly cloned Atlantic salmon (Salmo salar) PepT2. In Xenopus laevis oocytes, transient currents analysis showed that both total amount of charges moved (Q) and decay time (τ) vs membrane voltage shifted to more positive potential values when extracellular pH decreased, highlighting the role of H+ in the first step of transport cycle. Transport current vs voltage relations, as from Gly-L-Gln dose-response experiments, allowed kinetic parameters to be determined as a function of potential (from -140 to +20 mV) and external pH (5.5, 6.5, 7.6). Salmon PepT2 showed the higher apparent Gly-L-Gln affinity (K0.5) at pH 5.5 and 6.5 at the physiological membrane potential and an increase of maximal relative current (Imax) for more negative potentials and more acidic conditions. Notably, mRNA tissue expression analysis revealed that it is highly expressed in midgut and hindgut. Similar to zebrafish, this salmon PepT2 is a high-affinity/low-capacity transporter (K0.5 for Gly-Gln 4,4 μM, Imax -10 nA at -40 mV at pH 6.5), but its specific expression in the mid-to-distal portions of the gut opens to distinct and not yet known roles for a PepT2-type protein in fish physiology